Process and apparatus for descaling oxidized sheet metal

ABSTRACT

SHEET METAL IS SUBSMITTED TO AN EROSIVE SPARKING TREATINGMENT PRECEDING A CONVENTIONAL CHEMICAL DESCALING TREATMENT. THE SPARKING TREATMENT MAY REPLACE THE COVENTIONAL GRAINING TREATMENT. THE APPARATUS INCLUDES A SPARKING DEVICE ARRANGED UPSTREAM OF A CONVENTIONAL CHEMICAL DESCALING BATH AND COMPRISING A PLURAITY OF SPARKING ELEMENTS DISTRIBUTED OVER THE WIDTH OF BOTH FACES OF THE SHEET, AND A GENERATOR FOR FEEDING THE SAID ELEMENTS WITH CURRENT PLUSES PRODUCING ELECTRIC ARCS BETWEEN THEM AND THE SHEET-FACES. IT MAY ALSO COMPRISES A FEEDBACK CONTROL DEVICE FOR ADJUSTING THE FREQUENCY OF THE PULSES IN DEPENDENCE ON THE OBTAINED DEGREE OF DESCALING.

H. FICHAUX Jan. 23, 1973 PROCESS AND APPARATUS FOR DESCALIHQ. OXIDIZED SHEET METAL Filed Julv s. 1971 FIG.

W FIG. 4

FIG. 5 FIG. 6

- Henri FICHAUX Inventor 1! Hon: 2 y

United States Patent US. Cl. 134-1 13 Claims ABSTRACT OF THE DISCLOSURE Sheet metal is submitted to an erosive sparking treatment preceding a conventional chemical descaling treatment. The sparking treatment may replace the conventional graining treatment. The apparatus includes a sparking device arranged upstream of a conventional chemical descaling bath and comprising a plurality of sparking elements distributed over the width of both faces of the sheet, and a generator for feeding these elements with current pulses producing electric arcs between them and the sheet-faces. It may also comprise a feedback control device for adjusting the frequency of the pulses in dependence on the obtained degree of descaling.

It is known that one of the major difficulties encountered in the production of sheet metal, especially in the production of sheet steel, is the descaling operation, i.e. the removal of the oxide layer covering the outer surface of the sheet metal, after hot rolling, or in the case of cold rolling, after annealing. Although the outer portion of this oxide layer called calamine sometimes tends to peel off on cooling, a tough lower layer still remains on the surface of the sheet metal and this lower layer can only be removed by repeated electrochemical treatments which often must be preceded by a mechanical roughing treat ment followed by chemical finishing treatments. In practice, especially in the production of stainless sheet steel, the mechanical roughing treatment consists of a graining operation in which the sheet metal is bombarded with small grains of very hard material such as steel grains thrown at high speed against the sheet metal surface. Thi graining operation has two objects: First, it serves to remove the upper portion of the oxide layer and, second, it is intended to fissure the oxide layer which it does not succeed to remove, to thereby facilitate the penetration of the liquids used in the subsequent baths in which the following electrochemical treatments are carried out. In fact, without such fissuring the ubsequent electrochemical treatments would have to be longer or a greater number of them would have to be adopted. However, obviously it is more advantageous to reduce the number of the subsequent electrochemical treatments and also their duration, for these treatments are expensive as they necessitate the use of large basins which take up much space. Further, special precautions must be taken in carrying out these treatments as the liquid chemicals used are corrosive and often detrimental to health and they have to be replaced regularly because their descaling capacity decreases gradually in use.

Now, the efficiency of the graining operation cannot be increased beyond certain limits. Moreover, the impact of the grains on the oxide layer during the fissuring operation cannot be allowed to be too violent as the sheet metal, which, in a manner of speaking, performs the function of an anvil on which the oxide is hammered, would receive minute local deformations on its surface 3,712,833 Patented Jan. 23, 1973 ice which would make the surface rough and impair the brilliance of the finished product.

It is the object of the present invention to eliminate the aforementioned disadvantages by providing a descaling process which increases the efficiency of the electrochemical baths used in the conventional processes.

The process according to the invention is characterized in that the sheet metal band is subjected to at least one erosive sparking treatment by subjecting each of its surfaces to the impact of a plurality of electric arcs distributed over the entire width of said surface and repeated at a high frequency, and proceeding subsequently to at least one electrochemical and/ or chemical treatment of a conventional descaling process.

The invention also provides apparatus for carrying out this process, such apparatus comprising at least one sparking device arranged upstream of at least the first electrochemical or chemical treatment baths and formed of two halve each of which acts upon one of the two surfaces of the sheet metal band and includes an electrode extending over the width of the sheet metal band and formed of a plurality of projecting pointed sparking elements fed by a generator supplying a series of high frequency current pulses producing electric arcs in said sparking device.

A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawing, in which:

FIG. 1 is a schematic longitudinal section through an apparatus according to the invention;

FIG. 2 is an electric circuit diagram of one element of the apparatus;

FIG. 3 is an electric circuit diagram of another element of the aparatus;

FIG. 4 is an electric circuit diagram illustrating a portion of FIG. 3;

FIG. 5 is a diagram showing the arrangement of another portion of FIG. 3, and

FIG. 6 is a section showing a modification of a portion of the apparatus shown in FIGS. 1 and 3.

The process according to the invention essentially consists in that at least one sparking operation is included in the succession of conventional descaling operations. For this purpose each surface of the sheet metal band is subjected to the impact of a very rapid succession of sparks, i.e. small intermittent arcs, distributed over a plurality of points located closely side by side on a band covering the entire width of the sheet metal band. These sparks follow one another at a very high frequency which may reach several kilocycles. Preferably these sparks are produced successively at each point of said band so as to sweep all these points within a period shorter than the spark repetition period. Each of these points within a period shorter than the spark repetition period. Each of these sparks produces a current pulse having a very high peak intensity (up to several hundred amperes) so that these pulses form intermittent arcs whose impact on the sheet metal surface has a highly erosive effect. Due to the movement of the sheet metal band this repeated erosive effect is transmitted to different points of the sheet metal surface so that in the end the erosive effect extends over the entire sheet metal surface. The grade of descaling produced by this erosive effect can be varied by regulating the intensity of the peak current of the pulses and their repetition frequency. According to the invention this latter possibility is used to keep the degree of descaling substantially constant independently of the initial degree of oxidation which may vary strongly between different types of sheet metal. To effect this regulation the degree of descaling obtained is measured at the inlet to the electrochemical bath following the descaling operation and the arc repetition frequency is varied accordingly. This measurement may be performed by any appropriate means and a very practical solution consists in measuring the resistance of the sheet metal surface to the passage of electric current. This can be achieved, for example, by applying electric voltage of a predetermined height between the bath and the sheet metal and continuously measuring the intensity of the current passing at the interface between the sheet metal and the bath, the intensity of this current depending upon the amount of oxide present at this interface. Then the are repetition frequency is varied according to the value ascertained by this measurement. When the intensity of the current is weak, which indicates that a relatively thick oxide layer is still present, the arc repetition frequency is increased; when the intensity of the current is high, which indicates that only a thin oxide layer is left or none at all, the arc repetition frequency is reduced. Obviously suitable precautions must be taken to avoid disturbances in the resistance measurement which would have no relation to the oxide layer. For example, when measuring the current as described above, the electric conductivity of the liquid of the bath itself must also be taken into consideration to avoid disturbances due to variations of such.

conductivity caused by fluctuations of the temperature or concentration of the bath. However, this manner of measuring the degree of descaling by measuring the resistance is but one of several possibilities given merely by way of example and the degree of descaling could also be measured, for instance, by optical or acoustic or some other means.

By including in the succession of conventional descaling operations a sparking treatment carried out before the sheet metal band is subjected to the action of at least one electrochemical or chemical bath the efliciency of such bath or baths is increased. In fact, the sparking treatment erodes at least partially the oxide layer so that, even if it is not removed completely, its structure is changed, i.e. it is made porous, for each sparking arc impact produces a small local cavity or, in the most favourable circumstances, it pierces the oxide layer up to the metal. This porosity facilitates the penetration of the liquid in the following electrochemical or chemical bath. Thus, this sparking treatment before the introduction of the sheet metal band into the first bath, i.e. immediately after the graining operation, affords a great advantage. Under favourable conditions the graining operation may even be dispensed with and be replaced by the sparking treatment. In this case there is the additional advantage of reducing the roughness of the descaled sheet metal band. Thus, experiments made on a stainless steel band have shown that its roughness of about 0.6 to 0.8 mil produced by the conventional graining operation can be reduced to 0.2 mil by replacing the graining operation by the sparking treatment proposed by the present invention. This reduction of the roughness permits to obtain a sheet metal band having a very smooth surface and thus a higher quality.

However, even if the graining operation must be maintained, the introduction of a sparking treatment in the succession of the conventional descaling operations permits to greatly simplify this succession of operations by reducing the number of the baths required.

The apparatus for carrying out the process according to the invention is schematically illustrated in FIG. 1. FIG. 1 shows a sheet metal band 1 moving continuously in the direction of the arrow 2 and passing in front of a sparking device 3 and then over guide rollers 4 before entering an electrochemical or chemical bath 5. At the inlet of the bath 5 there is a measuring cell 6 for measuring the degree of descaling produced by the sparking device 3. The sparking device 3 and the measuring cell 6 are provided in pairs or two halves with one half associated with the upper surface of the sheet metal band 1 and the e he ha f atses at tl w th th lQwer s rfa e of this band. Each half is designed in the same manner as the other and therefore only one of them will be described hereafter, namely the one associated with the upper surface of the sheet metal band.

The sparking device 3 comprises an electrode 10 formed of a plurality of projecting pointed sparking elements 11 embedded in an insulating member 12. The upper end of each of the sparking elements 11 is connected to a distributor circuit 13 having its input connected to the main terminal 16 of a pulse generator 14 whose other main terminal 17 is connected to earth. The pulse generator 14 is designed to provide a succession of very short pulses of very intense current, these pulses following one another at a very high frequency whose value can be varied by applying signal to a control terminal 15 provided on the pulse generator 14. The distributor circuit 13 is designed to apply these pulses successively to each of the pointed sparking elements of the electrode 10, the sweeping of all the sparking elements being effected within a time shorter than a period of operation of the pulse generator 14.

The measuring cell 6 comprises a flat measuring electrode 20 immersed in the liquid 31 of a bath 5 at a predetermined distance from the surface of the sheet metal band 1. The electrode 20 is connected through a resistor 21 to one pole of an electric current source 22 supplying a predetermined voltage, the other pole of the electric current source 22 being connected to earth. The electric current source 22 also supplies current through a resistor 23 to a pair of auxiliary electrodes 24 immerged in the liquid 31 of the bath 5 adjacent the measuring electrode 20. The output voltage of the resistor 21 is fed to a comparator circuit 25 which also receives the output voltage from the resistor 23 inserted in the circuit of the auxiliary electrodes 24. The comparator circuit 25 is designed to provide at its exit 26 an output signal which corresponds to the difference between the voltages received from the resistors 21 and 23 and thus depends only on the size of the oxide layer covering the surface of the sheet metal band 1 and does not depend on the conductivity of the liquid 31 which may vary due to fluctuations in the temperature or concentration of the liquid. This liquid 31 is a corrosive liquid, for example, a solution of hydrochloric acid, capable of attacking the oxide layer on the surface of the sheet metal band 1.

As seen in FIG. 2, the measuring cell 6 forms a Wheatstone bridge having two branches one of which contains, in addition to the resistor 21, the amount of the liquid of the bath located between the sheet metal band 1 and the measuring electrode 20 while the other branch contains, in addition to the resistor 23, the amount of the liquid of the bath located between the auxiliary electrodes 24.

The exit 26 of the comparator circuit 25 is connected by a conductor 27 to the control terminal 15 of the pulse generator 14 so that the frequency of the latter is controlled by the output signal of the measuring cell 6. Thus, the conductor 27 is a feedback conductor which ensures that the pulse frequency of the pulse generator 14 is controlled in strict relation to the quality of descaling produced by the sparking treatment. This control permits to obtain a substantially constant degree of descaling even if the initial degree of oxidation varies. When the obtained degree of descaling is insufiicient the remaining oxide layer left on the surface of the sheet metal band 1 is thicker and the intensity of the current passing through the resistor 21 diminishes. As a result, the feedback signal transmitted by the conductor 27 produces an increase of the pulse frequency which in turn produces an increase of the descaling effect obtained by the sparking treatment. When the obtained degree of descaling is good the remaining oxide layer left on the surface of the sheet metal band 1 offers less resistance to the passage of the current through the resistor 21 and consequently the intensity of th s current increases A a esult, the feedback signal transmitted through the conductor 27 produces a reduction of the pulse frequency of the pulse generator 14, thus avoiding an excessive sparking treatment and a possible erosion of the very metal of the sheet metal. band which might result therefrom.

The corrosive liquid 31 may also be a solution of nitric acid which in certain cases produces a rapid passivation of the surface of sheet steel, which passivation increases in intensity in relation to the degree of descaling. Since this passivation tends to increase the resistance of the interface between the sheet metal and the liquid, the pulse generator 14 is in this case designed so that the output signal supplied by the comparator circuit 25 will produce in the pulse generator 14 an effect exactly inverse to what has been described above, namely so that the higher the resistance, indicating a good descaling, the lower will be the sparking frequency, whereas when the resistance is smaller, indicating a bad descaling, the sparking frequency will increase.

The pulse generator 14 may be of any conventional type provided that it is capable of producing power pulses of high intensity and following one another at a high frequency. However, as the electrode is located at a certain distance from the sheet metal band preferably a generator comprising two circuits is used, one of which generates a priming pulse of high voltage (of the order of several kilovolts), but of low intensity, capable of forming an ionized path for the power pulse of low voltage but high intensity to permit the power pulse to pass along this ionized path and produce the sparking arc.

FIG. 3 shows merely by way of example a working diagram of such a pulse generator. A pilot oscillator 40 is provided to control in parallel two circuits, i.e. a priming circuit 42 and a power circuit 43. The pilot oscillator 40 is arranged to oscillate at a predetermined frequency which can be adjusted by control means such as a control knob 41. The priming circuit 42 generates the priming pulse and the power circuit 43 generates the power pulse. These two pulses are successively distributed to the various sparking elements 11a, 111), etc. of the electrode 10 by the distributor circuit 13. As showing in FIG. 4, the distributor circuit 13 may be in the form of a delay line having a plurality of cells LC 32, 33, 34, etc. The coil L of each of these cells forms the primary winding of a transformer whose secondary winding is connected to one of the sparking elements of the electrode. For example, as shown in FIG. 4, the coil 38 of the cell 33 forms the primary winding of a transformer whose secondary winding 37 is connected to one of the sparking elements 11 of the electrode 10. The other ends of all secondary winding are connected to a common conductor connecting them to the exit of the power generator 43.

Such arrangement ensures that the output is not fed to only one sparking element and that each sparking element will produce an arc in its turn. However, the total delay caused by the delay line is considerably smaller than an oscillation period of the pilot oscillator 40 so that each pulse will have swept all the sparking elements before the arrival of the next pulse. The pilot oscillator 40 is so designed that its frequency may vary about the value established by the adjusting means 41, in response to a control voltage applied to a control terminal 44 which is the control terminal in FIG. 1 to which the feedback conductor 27 is connected.

The power circuit 43 is provided with means for adjusting the peak intensity of the current pulses that it delivers, this means being schematically shown in FIG. 3 as being constituted by a control knob 39.

Advantageously the pilot oscillator 40 may be provided with an auxiliary circuit which interrupts the operation of the priming circuit 42 after each priming pulse, the duration of this interruption being longer than the time required for deionization of the space between the electrode 10 and the sheet metal band 1. Such an auxiliary circuit ensures the extinction of the arcs formed below the sparking elements 11 and prevents the formation of continuous arcs.

The apparatus operates in the following manner:

The electric discharges occurring periodically between the sparking elements 11 of the electrode 10 and the surface of the sheet metal band 1 (FIG. 1) produce an erosive effect which removes at least part of the oxide layer from the portion of the surface of the sheet metal band located below the electrode 10. As the sheet metal band 1 moves in the direction of the arrow 2 its entire surface is progressively subjected to the descaling effect produced by this sparking treatment. The quality of this descaling effect is measured by the measuring cell 6 arranged in the position where the sheet metal band 1 enters the bath 5. The signal transmitted by the feedback conductor 27 causes a variation of the sparking frequency about the value set by the adjusting means 41 (FIG. 2) and ensures that this frequency is so adjusted that the quality of descaling produced by the sparking treatment will remain constant. The descaling effect is produced simultaneously on the upper and lower surfaces of the sheet metal band 1 since the sparking device 3 and the measuring cell 6 are provided in pairs or two halves with each half of the sparking device 3 being adjusted independently of the other by the corresponding half of the measuring cell 6.

As will be seen the sparking device 3 works about in the same manner as an electroerosion or spark machining device with substantially the only difference that the electroerosion process is carried out in a liquid environment whereas the sparking treatment proposed by the present invention is carried out in the air and that in a spark machining device the electrode is located very close to the workpiece whereas in the sparking device according to the invention the electrode is spaced from the sheet metal band by a substantial distance (about 0.04 to 0.4 in.). This spacing is the reason why a priming discharge is used to provide the ionization required for the passage of the sparking arc.

Another embodiment of the sparking device is shown in FIG. 6 wherein the solid sparking elements shown in FIGS. 1 and 3 have been replaced by hollow sparking elements. Such a hollow sparking element as shown in FIG. 6 has the form of a cylindrical bar 51 provided with a bore 52 closed at its lower end by an end wall 53 provided with a central circular hole 54. The bore 52 accommodates a priming stem 55 having a lower end 61 received in the hole 54 at the lower end of the bar 51. At the upper end of the bar 51 the priming stem 55 projects from the bore 52 through a transversely extending insulating member 56. A tube 57 is secured to the bar 51 near its upper end and communicates with the bore 52 therein. The tube 57 is connected to a source of compressed gas (not shown), for example, air or a reducing gas, to permit a continuous stream of such gas to be fed under pressure into the bore 52 from which it escapes through the annular hole 54.

The priming stem 55 is connected to one end of a secondary winding 58 of a transformer 59 with the other end of this secondary winding connected to the bar 51. The bar 51 is connected to the power circuit 43 (FIG. 4). The primary winding 60 of the transformer 59 is the coil L of one of the cells LC of a delay line forming a distributor circuit similar to that shown in FIG. 4 so that the transformer 59 works in the same manner as the transformer 37, 38 shown in FIG. 4. When an output pulse is supplied by the priming circuit 42 the transformer 59 produces a high voltage between the cylindrical bar 51 and the priming stem 55 and this high voltage causes a disruptive discharge betwen the lower end 61 of the stem 55 and the lower end wall 53 of the bar 51. This discharge brings about a strong ionization within the annular portion of the hole 54 and, as there is a continuous flow of gas through this annular portion of the hole 54, the ionized gas is blown out of the bar 51 and forms a beam 62 at the lower end thereof. This beam is formed of a conductive medium which forms a sort of extension to the bar 51 to thereby extend the sparking element and artifically shorten the distance between the latter and the sheet metal band 1. As a result of this shortened distance between the sparking element and the sheet metal band the voltage supplied by the generator 43 to the sparking element is suflicient for the formation of the sparking are.

This modified embodiment has the advantage that the sparking element is efficiently cooled and that the oxide particles broken away by the sparking are are removed by the fiow of gas from the hole 54. Moreover, the blowing effect produced by this flow of gas permits the arc repetition frequency to be increased for, by continuously replacing the air in the vicinity of the sparking element, it positively removes the ions in a much more rapid manner than would be possible merely by recombination ionization.

Besides, the advantages of the blowing effect can also be utilized in connection with an electrode having solid electrode elements as the electrode 10 shown in FIGS. 1 and 3. For this purpose it is sufiicient to provide this electrode with a compressed gas supplying device which produces a flow of gas directed laterally of the electrode in parallel to the sheet metal band, preferably in a direction opposite to the direction of movement of the band.

The sparking device 3 shown in FIG. 1 is located upstream of the bath 5. However, since a descaling apparatus generally comprises a plurality of electrochemical and/or chemical baths arranged one behind the other, it may be desirable to provide further sparking devices arranged upstream of one or the other following baths. At any rate, the apparatus according to the invention will always comprise a sparking device arranged upstream of the first bath and with certain types of sheet metal this sparking device may even replace the conventional graining device which may be dispensed with. In this manner the apparatus is considerably simplified. If in some cases the graining device must be maintained, the sparking device is placed between the graining device and the first electrochemical or chemical bath. In this manner the graining device will produce a rough descaling and the sparking treatment will produce a fine descaling while the electrochemical and/ or chemical bath or baths will produce a finishing descaling effect.

The apparatus proposed by the present invention can generally be used for all ferrous and non-ferrous metals which tend to oxidize at a certain phase of their pro duction, for example, during hot rolling or during an nealing after cold rolling. However, this apparatus is primarily intended for working ferrous sheet metals, particularly weak alloy stainless steel sheets. Thus, with a ferrous metal sheet containing 17% of chromium, covered with an oxide layer having a thickness in the order of 0.4 mil, an excellent descaling effect has been obtained, that is to say, the metal surface has been completely uncovered by subjecting the sheet metal band 1 to a sparking treatment with an electrode 10 formed of electrode elements having a diameter d of 0.4 in. (FIG. and distributed within the electrode so that the electric power lines 46 produced thereby and sweeping the surface of the sheet metal band 1 were spaced by a distance 2 of 0.08 in. The electrode itself was placed at a distance of 0.12 in. from the surface to be descaled and was fed by a generator producing pulses of a peak current intensity in the order of 500 a., a duration in the order of 1 ms. and a frequency of 200 Hz. while the sheet metal band moved at a speed of 12 in./sec.

What I claim is:

1. A process for descaling an oxidized sheet metal band, characterized in that the sheet metal band is subjected to at least one erosive sparking treatment by subjecting each of its surfaces to the impact of a plurality of electric arcs distributed over the entire width of said surface and repeated at a high frequency and the sheet metal band is subsequently subjected to at least one chemical treatment of a conventional descaling process.

2. A process as claimed in claim 1, wherein the chemical treatment is electrochemical.

3. A process as claimed in claim 1, wherein the sheet metal band is subjected to a graining treatment before carrying out the erosive sparking treatment.

4. A process as claimed in claim 1, wherein the quality of the work produced by said erosive sparking treatment is continuously controlled by measuring the quantity of oxide remaining on the sheet metal band after said treatment and by adjusting the spark repetition frequency in relation to the result of said measurement to maintain said quality constant.

5. A process as claimed in claim 4, wherein said measurement is effected at the inlet to the chemical bath in which the sheet metal band is treated after the erosive sparking treatment.

6. A process as claimed in claim 5, wherein said measurement is effected by measuring the electrical resistance at the interface between the sheet metal band and the liquid of said bath.

7. Apparatus for descaling an oxidized sheet metal band, including at least one chemical descaling bath, characterized by a sparking device arranged upstream of at least the first chemical treatment baths and formed of two halves each of which acts upon one of the two surfaces of the sheet metal band and includes an electrode extending over the width of the sheet metal band and formed of a plurality of projecting pointed sparking elements fed by a generator supplying a series of high frequency cur rent pulses producing electric arcs in said sparking device.

8. Apparatus as claimed in claim 7, wherein the bath is an electrochemical descaling bath.

9. Apparatus as claimed in claim 7, wherein the electrode is provided with a compressed gas supplying device to blow compressed gas in a direction parallel to the surface of the sheet metal band.

10. Apparatus as claimed in claim 7, wherein each sparking element of the electrode is formed of a conductive bar provided with a bore communicating with the atmosphere through an axial hole provided at one end of said bar, a conductive stem extending axially along said bore through an insulating member at the other end of said bar, the free end of the conductive stem being located in the center of said axial hole, and a tube connection located near one end of said bar and communicating with a source of compressed gas to produce a gas flow though said bore and axial hole in a direction perpendicular to the surface of the sheet metal band, said generator including a priming circuit for producing between said stem and said bar a succession of high voltage pulses generating priming sparks between the free end of the stem and the surrounding edge of the axial hole, and a power circuit for transmitting said high voltage pulses to said bar.

11. Apparatus as claimed in claim 7, wherein a measuring device is associated with the sparking device to provide a control signal representing a physical value indicative of the quantity of oxide left on the sheet metal band after having passed through the sparking device, said measuring device being connected to the pulse generator of the sparking device through a feedback conductor and said pulse generator being so designed that its frequency will depend upon the control signal produced by the measuring device so that the quantity of oxide left on the sheet metal band will remain constant independently of the quantity of oxide covering said band before its passage through the sparking device.

12. Apparatus as claimed in claim 11, wherein the measuring device includes a measuring cell operating in response to he ele t ic e is c at t e i te ce b t en.

the sheet metal band and the liquid of the chemical bath and this electric resistance constitutes said physical value.

13. Apparatus as claimed in claim 12, wherein the measuring cell comprises a source of electric current to supply current of a predetermined voltage, a measuring electrode immerged in the liquid of the chemical bath in parallel to and at a short distance from the sheet metal band, a pair of auxiliary electrodes immerged in the liquid of the same bath in the vicinity of the measuring electrode, a first resistor connected between the measuring electrode and the source of current, a second resistor connected between the auxiliary electrodes and the source of current, and a comparator circuit having an inlet connected between the measuring electrode and said first resistor and another inlet connected between the auxiliary electrodes and said second resistor and an outlet connected to the pulse generator, these members being connected so as to form a measuring bridge fed by said source of current and including in one of its branches one of said resistors, the sheet metal band, and the amount of liquid between the sheet metal band and the measuring electrode, and in the other branch the other resistor and the amount of liquid between the auxiliary electrodes, said comparator circuit being connected between the two ends of the diagonal of the measuring bridge so that the output signal obtained therefrom depends upon the value and direction of the voltage in this diagonal and constitutes the control signal for the pulse generator.

References Cited UNITED STATES PATENTS 1,897,963 2/1933 Walker l34-l 2,009,213 7/1935 Walker 134-1 MORRIS O. WOLK, Primary Examiner D. G. MILLMAN, Assistant Examiner U.S. Cl. X.R. 

